1. Field of the Invention
The present invention relates generally to thermosetting resins and the many different types of compositions that contain such thermosetting resins. More particularly, the invention involves those types of thermosetting resins and compositions that are stored at ambient temperatures and then cured by adding a curing agent and increasing the temperature of the resin to a curing temperature that is relatively close to the storage temperature. The present invention involves providing resins that have a relatively high viscosity at ambient storage temperatures and are suitable for use as adhesives and in prefabricated uncured composites known as prepreg. The resins are converted to a low viscosity material when heated to curing temperatures to provide a rapid flow of the resin that may be required for adequate mixing with curing agents and/or penetration into porous bodies such as fiber bundles and fabric.
2. Description of Related Art
Thermosetting resins are used as a principal ingredient in a multitude of different materials. For example thermosetting resins are widely used alone or in combination with certain additives to form adhesives. Thermosetting resins are also combined with a wide variety of fiber types and configurations to form composite materials. Epoxy resins, vinyl ester resins and cyanate ester resins are examples of thermosetting resins that have been in widespread use for many years.
The curing procedure for thermosetting resins typically involves adding one or more curing agents to the uncured resin to form an activated resin. The activated resin is then usually heated for a sufficient time to completely cure the resin. In many situations, it is desirable to prepare the activated resin and then store it for later use. During storage, the activated resins must be kept at temperatures that are well below the curing temperature of the resin in order to avoid premature curing. For this reason, many activated thermosetting resins cannot be stored at ambient temperatures. Accordingly, it has been common in the past to keep such activated resins refrigerated until they are ready to be used.
Composite materials are used extensively in situations where high strength and low weight are desired. Composites typically include fibers and a resin formulation as the two principal elements. A wide range of fiber types, sizes and orientations have been used in composites. Glass, graphite, carbon, p-aramid, m-aramid, quartz, thermoplastic, boron, ceramic, and natural fibers are common. The fibers can be chopped, stretch broken, randomly oriented, unidirectional in orientation or woven into fabric. The fibers used in composite materials have diameters that range from extremely small to relatively large. Although it is possible to make composites using large diameter fibers, the more common practice is to take thousands of fibers having extremely small diameters and form them into individual bundles known as tows. These multi-fiber tows are much stronger and more flexible than single fibers having the same overall dimensions. Filament bundles can have a wide variety of cross-sectional shapes including ellipsoidal, kidney and pea shapes. The tows can be woven into fabric in the same manner as conventional yarns. Alternatively, the tows are arranged in parallel to provide a unidirectional fiber orientation or they can be randomly oriented.
Thermosetting resins have been widely used as the resin matrix in composite materials. There are a number of ways to combine the resin with the fibers to form the final composite. One approach that has been used for years is to impregnate the fibers with activated resin and allow the resulting “lay-up” to cure at room temperature. The cure time is usually reduced substantially by heating the lay-up. This type of process is well suited for use in the field. However, this wet lay-up process has a disadvantage in that it is difficult to accurately control the amount of resin that is applied to the fibers and ensure that the resin is being uniformly impregnated into the fibers. In addition, the amounts of curing agent and other additives that are added to the resin may vary between lay-ups.
In order to avoid the above problems, it has been common practice to form prefabricated lay-ups that include fibers, resin and curing agent. These prepregs are made under manufacturing conditions that allow the amount and distribution of resin and curing agent within the fibers to be carefully controlled. The prepregs are typically refrigerated during storage and shipping to prevent premature curing of the resin matrix. The need to refrigerate prepreg presents a number of problems. It is expensive to store and ship prepreg on a commercial level because large refrigeration units are required and refrigerated trucks must be used. In addition, the temperature of the prepreg must be continually monitored to detect any increase in temperature due to equipment failure or the like. Increases in temperature, even for short periods of time, can adversely affect the shelf life and function of the prepreg and result in the prepreg being discarded.
One approach to eliminating the need for refrigeration of prepreg involves placing the resin and curing agent in the prepreg structure so that they are physically separated from each other. For example, the resin and curing agent can be located on opposite sides of a layer of woven fabric to form a prepreg that can be stored indefinitely at room temperature as described in U.S. patent application Ser. No. 648,159. When ready for use, the prepreg is heated, usually under pressure, so that the resin and curing agent flow into the fabric to initiate the curing process. The basic approach used in these types of systems is to store the resins and curing agents as separate entities that are in sufficiently close proximity to each other so that they can be mixed together by heating. This type of approach can also be used for thermosetting adhesives and other applications where the structure of the system allows the resins and curing agents to be kept in close proximity to each other without contact. Such systems typically include a porous body of some type that provides the structure in which the resin and curing agents are located.
There are a number of desirable properties that the resins and curing agents should have in order to be used in the ambient temperature storage systems described above. For example, the resin should be sufficiently viscous at room temperature so that it does not flow to any appreciable extent into contact with the curing agent. At the same time, the resin must retain sufficient tackiness and other properties that are desirable in a prepreg. The resin should be convertible to a relatively low viscosity material when heated to provide rapid and thorough mixing of the resin and curing agent. The change in resin viscosity should occur at temperatures that are relatively close to room temperature. For example, the viscosity change should preferably occur within 10° C. to 60° C. above ambient temperatures.
There is a present and continuing need to develop resins that are suitable for use in prepreg and other systems of the type described above that can be stored at ambient temperatures. In addition, there is a present and continuing need to develop prepreg and other system configurations that include resin/curing agent combinations that can be stored at ambient temperatures while still demonstrating the ability to undergo efficient cure at temperatures not significantly higher than ambient temperature. Cure temperatures below 100° C., more preferably below 80° C., and most preferably as low as 60° C. are of increasing interest to resin and/or prepreg converters because the use of these temperatures offers significant benefits in terms of energy consumption. Furthermore, as the cure temperature is decreased the processing equipment needed to cure the epoxy resin formulations becomes somewhat simpler and less expensive. For example, it becomes possible to use temporary, bespoke curing ovens constructed using inexpensive, but relatively temperature-sensitive components such as wood and polyolefinic sheeting. The resins should undergo relatively large reductions in viscosity over relatively small increases in temperature to provide thorough mixing of the resin and curing agent as well as uniform distribution of the resin throughout the cured structure.